The year-end 2008 PGC potential natural gas resource estimate, which is the volume the Committee believes could be found and produced in future years, is comprised of 1,673 Tcf resident in traditional gas reservoirs and 163 Tcf resident in coalbed reservoirs. Estimated traditionally reservoired resources are 519 Tcf (21.6%) higher than in 2006, whereas estimated coalbed methane resources are 3 Tcf (1.9%) lower. The changes arose primarily from analysis of new data for the Gulf Coast, Rocky Mountain, Atlantic, and Mid-Continent areas, which jointly account for 87% of estimated traditional resources. The largest increase in gas resources resulted from new assessments of shale gas plays located in the Atlantic area's Appalachian Basin, the Mid-Continent area's Arkoma and Fort Worth basins, several Gulf Coast area basins, and the Rocky Mountain area's Uinta Basin. Shale gas now comprises 616 Tcf (33%) of potential natural gas resources.